The present invention relates to a method for controlling and optimising the cycle for production of ice cream depending on the mixtures used.
At present the production cycle of an ice-cream making machine is controlled depending on the hardness achieved by the ice-cream mixture, said hardness being detected by mechanical means or by recording the power consumption of the stirrer motor.
However, this method of control does not take into due consideration factors such as the composition of the ice-cream mixture (mixture based on fruit or milk or fat) or the quantity of mixture present in the mixing chamber, or also the quantity of liquid mixture fed into the mixing chamber following extraction of a considerable quantity of ice cream, etc.
The main subject of the present invention, therefore, is a method which allows the preparation, within a predetermined freezing time, of an end product (ice cream) with repetitive characteristics as regards its consistency and its temperature, the quantity of emulsified air (overrun) and its creaminess, irrespective of the quantity and the type of product being processed in the freezing cylinder of the machine.
This result is obtained by suitably managing the operation of the compressor, regarded as a cold-generating source, of the stirrer/scraper regarded as a means for distributing cold throughout the mass of the product, and of the valve for injecting hot gas into the evaporator, regarded as a means for controlling the temperature of the refrigerating fluid in the said evaporator.
The management of these components is aimed at maximising the coefficient of transfer between the refrigerating fluid and the product, preventing the evaporation temperature between inlet and outlet of the evaporator from falling below a predetermined value, causing the formation of ice on the surface of the freezing cylinder (product side) and a consequent reduction in the flow of heat from the product to the refrigerating fluid, with an undesirable increase in the freezing time.
The management of these components is performed automatically by the machine designed so as to be capable of recognising both the quantity of product present in the evaporator chamber and the quality thereof, namely whether it consists of a watery fruit-based mixture or creamy milk-based mixture.
The quantity of product is determined by the machine on the basis of the progression of the integral or sum of the temperature differences of the refrigerating fluid between the evaporator inlet and outlet. This reading is performed at regular time intervals, for example every 4″.   INTEGRAL  =            ∑      →      0        ⁢          Δ      ⁡              (                              T            U                    -                      T            E                          )            
The quality of the product is recognised by the machine on the basis of the number (Nq—product quality number) obtained by multiplying the abovementioned integral, revised at each subsequent reading, by that which is defined here as the “consistency derivative” of the product recalculated at each reading, the term “consistency derivative of the product” being understood as being the ratio between the increase in consistency Δc in a time interval Δτ which is very short and the time itself:       Consistency    ⁢                   ⁢    derivative    =            Δ      ⁢                           ⁢      c              Δ      ⁢                           ⁢      τ      
There exists, in fact, for each machine a value Nm which can be determined in an experimental manner such that, if the number obtained as described above is greater than it, the product inside the chamber is of the creamy type, whereas if it is lower than it, the product inside the chamber is of the watery or fruit-based type.
The product of the watery type is most sensitive to low evaporation temperatures of the refrigerating fluid since, in these conditions, it forms more easily than the creamy product layers of ice on the inner wall of the freezing cylinder, assuming a characteristic appearance, with loss of “creaminess”, which is referred by experts in the sector as “frozen” owing to the presence of large ice crystals in its mass. In these conditions, since the layer of ice on the cylinder wall acts as a heat insulant delaying the exchange of heat between evaporator and product, the product being processed takes a relatively long time to reach the optimum consistency required and, on many occasions, does not even reach it.
In order to overcome this problem, it is envisaged according to the present method injecting into the stream of the refrigerating fluid, at the inlet of the evaporator, a suitable quantity of hot gas taken from the delivery of the compressor. It is pointed out that these injections of hot gas per unit of time must not be too numerous to avoid reducing excessively the life of the valve and must each be sufficiently short not to modify substantially the temperature condition of the refrigerant in the evaporator.
Since the evaporator reacts to a temperature change at its inlet with a delay of about 60 seconds, the hot gas injections must cover a much shorter period, equal to about 20 seconds, and have a duration of 0.5 to 2 seconds.
The hot gas is therefore supplied continuously at the start of the freezing time so as to establish an “operational zero” of the evaporator distinguished by a predetermined value of the temperatures of the refrigerant at the inlet and at the outlet of the evaporator in the presence of “zero consistency” and as soon as the inlet temperature drops below +10° C. and the outlet temperature drops below +21° C.
When the operational zero is reached, the supplying of hot gas continues for the whole of the freezing period for the durations indicated above.